A spouse is added to a marriage, not directly to the individual. So to add a spouse you first edit the marriages of the individual. Clicking on the "Edit" button associated with marriages on the individual edit dialog for the existing individual, pops up the Edit Families dialog. This dialog lists all existing marriages/relationships for the individual, with the associated date and the name of the spouse. If no spouse has been associated with the marriage, the spouse's name appears as '? ?'. You can choose to edit such a marriage/relationship, or to add a new marriage/relationship by clicking on either an existing "Edit Family" button, or on the "Add Family" button. Either button pops up an Edit Families dialog. You use this dialog to identify the spouse you wish to add, either by choosing an existing individual, or adding a new individual to the family tree.